Cardiac arrhythmias such as atrial fibrillation, bradycardia, ventricular tachycardia, ventricle fibrillation, and Wolff-Parkinson-White syndrome are common heart abnormalities that cause stroke, myocardial infarction, and other thromboembolic events within the body. In patients with normal sinus rhythm, the heart is electrically excited to beat in a synchronous and patterned manner, typically at a rate of 60 to 100 beats per minute (bpm). In contrast, in patients with cardiac arrhythmia, abnormal regions of the cardiac tissue may aberrantly conduct to adjacent tissue, causing the heart to beat irregularly. In ventricular tachycardia, for example, electrical signals may be errantly received in the lower heart chamber (i. e. the ventricle) instead of the right, upper chamber (i.e. the atria), causing the heart to beat rapidly. In atrial fibrillation, the most common type of cardiac arrhythmia, the upper chambers of the heart beat at an uncontrolled rate of 350 to 600 bpm, which results in a reduction of the pumping force of the heart. As a result of this reduced pumping force, blood in the heart chambers may become stagnant and pool, forming blood clots that can dislodge within the body and cause stroke or other life threatening events.
To treat cardiac arrhythmia, a number of therapeutic procedures have been developed, including RF catheter ablation, chemical cardioversion, percutaneous myocardial revascularization (PMR), and suppression. Antiarrhythmic medications such as beta-blockers, calcium channel blockers, anticoagulants, and DIGOXIN have also been used successfully to treat some forms of cardiac arrhythmia. More recent trends have focused on the use of cryotreatment catheters to treat arrhythmias such as atrial fibrillation and ventricular tachycardia. Such devices provide a relatively non-invasive method of treatment in comparison to other surgical techniques.
In one such method, for example, a catheter loaded with a cryogenic cooling fluid may used to cryogenically cool cardiac tissue at strategic locations of the heart, such as the right and left atria, or the pulmonary veins. The catheter can be used to induce necrosis at one or more pre-mapped target sites within the heart to create conduction blocks within the aberrant electrical conduction pathways. In atrial fibrillation, for example, necrosis of one or more target sites within the atrial cardiac muscle tissue can be used to block the electrical signals believed to cause and/or sustain the fibrillation.
In some techniques, the use of a cryotreatment device to form the required conduction block may be ineffective since there is no adequate means to control the transmural depth of the lesion, or the distance between each lesion. To compensate for these shortcomings, many cryotreatment devices utilize relatively large catheter tips, which destroy more tissue than is necessary to form the conduction block and further reduce the already weakened pumping force of the heart. It is therefore desirable to have a cryotreatment device capable of transmurally controlling the depth of each lesion and in a contiguous manner.